The use of ceilometers which employ a laser instead of more conventional light sources such as pulsed arc-discharge lamps inherenty can make possible a very substantial increase in the distance range within which such ceilometers can operate, due to the higher transmittable powers and lower beam divergence which are characteristic of lasers. However, very serious limitations are placed upon the transmitted-power increases potentially available, due to considerations involving the possibility of damage to the eyes of human observers. Internationally accepted safety standards intended to prevent the possibility of eye damage to human observers, require, as a matter of governmental regulation, that the transmitted radiant power be limited to what are indeed relatively low levels, compared to those potentially available. These governmental regulations take into account not merely the possibility of eye damage resulting to an observer who looks into the emitted beam with his naked eye, but additionally take into account the possibility for eye damage when the beam is inadvertently looked into by a human observer using various optical instruments, e.g., a pair of binoculars, a telescope, and so forth. For example, in the U.S., the ANSI standards (in Class 3) require that no eye damage result when an emitted pulse of laser energy is viewed through a telescope whose objective has a diameter of 80 mm.
The expression "viewing emitted laser energy" will be understood to refer to the case where a human observer looks towards the laser radiation source or the transmitting optics of the ceilometer in the direction exactly opposite to that in which the laser radiation is being emitted from the ceilometer. Likewise, it will be understood that laser radiation of the types which can be employed for ceilometric applications can be of wavelengths falling outside the range of visible wavelengths.